One object of image acquisition devices, such as Xerographic copying, scanning, and printing systems, is to output images having colors and densities that have a direct correlation to the colors and densities of the input images. A common technique for monitoring the quality of output images is to utilize one or more test patches of predetermined densities. The actual density of the printing material, toner or ink for example, in a test patch is optically measured to determine the effectiveness of the image acquisition device to produce correct densities in output images.
Densities of input images are sensed by sensors such as densitometers. A densitometer is a device for determining the density of print material on a surface, such as a visible-light densitometer, an infrared densitometer, an electrostatic voltmeter, or any other device which makes a measurement from which the density of print material may be determined. A reflection densitometer detects the optical density of an image portion by sensing the amount of light reflected from the image portion in relation to the amount of light impinging on the image portion. In Xerographic image acquisition devices such as scanners, the sensor that detects the image is often a charge coupled device (CCD).
The optical density of an image portion is equal to the absorbance of the image portion, which is defined as log10(I0/I) where I0 is the intensity of light incident on the image portion, and I is the intensity of the light transmitted through the image portion. Although absorbance technically has no units, absorbance or optical density is sometimes expressed in “absorbance units” or AUs.
Sensors such as CCDs in scanners are generally calibrated at the time of manufacture by the use of a set of tone patches of varying and known densities. Because the sensors used in scanners often do not exhibit a linear relationship between input density values and output sensed density values, a set of tone patches that includes more than two tone patches often is used so that non-linear higher-order correction can be implemented. For example, a set of tone patches for calibrating a scanner often includes five tone patches. A common density configuration for sets of five tone patches is 0.1, 0.3, 0.6, 1.2, and 1.5.
In operation, the tone patches are sensed by the scanner's sensor and sensed density values are output by the scanner's sensor. These sensed density values and the known, correct density values for the tone patches are used to create a tone reproduction curve (TRC) for the scanner's sensor. A TRC represents the correspondence between density values sensed and Output by a scanner's sensor and the correct density values. That is, the TRC linearizes the sensor's output. Because only a finite number of tone patches are sensed in the initial calibration, a TRC is generated by interpolation and extrapolation such as by polynomial matching. Commonly, the resulting TRC is sampled and stored as a look-up table (LUT) in the memory of the scanner. The original TRC is usually curved as shown by TRC 100 in FIG. 1.
With time and use, such as in a customer environment, the correction afforded by the TRC becomes less accurate. This occurs from various causes, including changes in the intensity and frequency distribution of the luminous output of the scanner's lamp and changes in the output characteristics of the scanner's sensor. In response, methods have been implemented to recalibrate the output of scanners.
One method that has been used is to recalibrate the scanner, such as at the factory or in the field, by use of a tone patch set having the same, or a similar, number of tone patches as the tone patch set that was initially used to calibrate the scanner. This method has the disadvantages that it is necessary to ship the scanner to a service facility for service or to have a technician come to the scanner, necessitating that the scanner remains uncalibrated and, possibly, unavailable for use for an extended period of time. Further, either external equipment is required to calculate the new TRC, or additional processing power and memory are needed in the scanner to carry out the processing. Hardware able to carry out the necessary processing and able to store the necessary values to calculate a TRC is often more expensive than the hardware necessary for normal operation of a scanner. For at least these reasons, this method typically is not used because it is expensive, time consuming, and can incur significant down time for the scanner.
Another method is to service the scanner by replacing those parts, such as the lamp or image sensor, that have developed deviations over time. However, this method is expensive and can incorporate significant delays before recalibration. Further, unless the replacement parts accurately match the original performance characteristics of the original parts used to calculate the original TRC, this method does not generally produce significant improvement.
In another method, the scanner is recalibrated in the field by use of a calibration strip in the scanner. The calibration strip generally contains two patches of different densities, usually at the extremes of the density range, such as 0.1 and 1.5. These patches of the calibration strip are sensed by the scanner's sensor, which outputs sensed densities for the tone patches. The scanner then recalibrates itself based on the sensed densities and the known densities of the tone patches. Because only two tone patches are commonly used, however, in this situation the scanner is only able to generate a new TRC that is linear. That is, the TRC has the form of yi=g*xi+b and is a straight line as shown by TRC 200 in FIG. 1. Because TRC curves exhibit non-negligible high-order curvature, as shown in TRC 100 FIG. 1, a linear TRC approximation, such as TRC 200, is not accurate enough for many applications.
Additionally, further to the last method of recalibrating a scanner, the calibration strip included in the scanner can yellow or otherwise deviate from the known densities over time. Thus, recalibration techniques that use the calibration strip will incorporate the deviations from the expected values exhibited by the calibration strip from the known density values.